The present invention relates generally to the sensor art and, more particularly, to a sensor for sensing the location of a physical characteristic of a workpiece on which an automated process or like operation is being performed.
In automated welding systems, the ability of the welder to reliably track a seam or other physical characteristic associated with a workpiece is a fundamental and exceedingly important requirement. Various types of contact and non-contact sensors have been proposed to locate the weld seam and/or measure the weld joint. However, non-contact sensors are generally preferred for most applications, since interference with the workpiece is avoided.
In the past, others have proposed various types of non-contact sensors, including for example xe2x80x9cthrough-the-arcxe2x80x9d and machine vision sensors. In xe2x80x9cthrough-the-arcxe2x80x9d sensors, the location of the seam or other physical characteristic of the workpiece is determined by evaluating a characteristic, such as the arc length, of a transferred electrical arc formed between a welding torch, or more specifically the electrode associated therewith, and the workpiece. One example of a transferred arc sensor is found in U.S. Pat. No. 4,531,192 to Cook, the disclosure of which is incorporated herein by reference.
Transferred arc sensors have a few desirable characteristics when used in practical automated welding systems. For instance, sensing function is generally unaffected by the light projecting from the arc, heat radiation, and spattering. Another desirable characteristic is that no specialized hardware is required, since the transferred arc produced by the welding torch or electrode itself provides the sensing function. This keeps manufacturing and operating costs relatively low. In addition, the performance of a transferred arc sensor does not, in general, depend on either the surface conditions or the material forming the workpiece.
Despite these beneficial characteristics, transferred arc sensors do have some significant limitations and shortcomings. For example, to locate the seam, transferred arc sensors must xe2x80x9cweavexe2x80x9d to and fro across the workpiece. Since the sensing function is provided by the same arc used to weld the workpiece, this weave creates interference with the welding process and may not always be permissible or desirable. Also, for welding processes employing a consumable electrode, such as gas metal arc welding (GMAW) the arc length generally varies due to the continuous transfer of droplets of molten metal from the end of the electrode. This variable arc length creates a highly variable arc voltage signal, which of course significantly complicates locating the weld seam.
Further, due to the distribution of the electric welding arc in conventional GMAW or similar welding techniques where a transferred arc is established between the welding torch and the workpiece, the tracking resolution is usually relatively great (i.e., one millimeter or greater). Moreover, in a transferred arc arrangement, the anode spot (that is, the spot where the transferred electrical arc makes contact with the workpiece) tends to move, or xe2x80x9cjump,xe2x80x9d from place to place because of the minimum arc principle. Of course, this xe2x80x9cjumpxe2x80x9d makes it difficult to locate the edges of small seams having narrow root openings with any degree of precision.
Compared with transferred arc sensors, machine vision sensors employing cameras are less process-dependent and create less interference with the welding process. A high tracking resolution is also possible, depending on the field of view and type of camera used. In addition to seam tracking, the camera may also be used to obtain detailed information about the profile of the joint geometry for use in advanced process control.
However, machine vision sensors are quite sensitive to environmental conditions, such as the light projecting from the arc, metal spatters, surface shine of the workpiece, and orientation. Also, the hardware required, including the camera, is expensive and the maintenance costs are relatively high. All of these shortcomings make machine vision sensors generally undesirable for use in basic automated welding operations.
Accordingly, a need is identified for an improved sensor for sensing a characteristic of a workpiece, such a weld seam, in an automated process, such as a welding operation. The sensor would provide reliable sensing function despite the presence of arc light, heat radiation, spatters, surface shine, or variations in the type of material forming the workpiece. In automated welding, the sensor would operate independently of the welding process itself, thereby avoiding the problems associated with conventional transferred arc sensors. The sensor would also have sufficient accuracy/tracking resolution to identify the seam for many different joint designs, including the common square butt joint, as well as to precisely locate edges, holes, inclines/declines, etc. The sensor would also use existing technology, would be inexpensive to produce, implement and maintain, and would also have a reasonably long service life.
An apparatus, system and related method are disclosed for sensing a characteristic of a workpiece, such as the location of a welding seam, using a sensor that is independent of another device for performing an operation on the workpiece, such as a welder. In the most preferred embodiment, a plasma arc torch serves as the sensor. This torch travels along the workpiece during the automated welding operation, preferably just ahead of the welder for welding a seam formed in the workpiece, and moves to and fro in a direction substantially transverse to the direction of travel of the welder. A non-transferred electrical pilot arc is established on the plasma arc torch, such as the electrode and a constricting nozzle in the conventional plasma arc welding arrangement. As is known in the art, this non-transferred arc serves to ionize a plasma gas issuing from the torch to create a concentrated plasma jet that is capable of conducting current. This plasma jet is directed towards the workpiece, usually by an orifice in the constricting nozzle. By sensing a characteristic of the plasma jet as the workpiece is traversed, such as the change in voltage across the sensing circuit that includes the plasma jet as a resistive element, the location of a seam or other physical characteristic (i.e., a hole, plate, or the like) may be determined. As should be appreciated, the stiffness and concentration of the constricted plasma jet significantly improves the tracking resolution, making it possible to sense narrow seams having small root openings (i.e., one millimeter or less). Also, since no electrical arc is transferred, no anode spot is created on the workpiece. Thus, deleterious anode spot xe2x80x9cjumpingxe2x80x9d is avoided and the dependence on the minimum arc principle is eliminated. Also, the proposed sensor advantageously operates independently of the welding process, and thus can be applied to various welding processes, such as arc welding, including plasma arc welding and short-circuiting transfer, without significant modification of the basic hardware and software employed.
In accordance with a first aspect of the present invention, an apparatus for sensing a physical characteristic associated with a workpiece is provided. The apparatus includes a sensor for directing a plasma jet toward the workpiece and a first motive device for moving the sensor relative to the workpiece in a first direction. The physical characteristic of the workpiece is sensed by observing changes in a reference characteristic of the plasma jet. Based on this sensed physical characteristic, an automated process, such as a welding operation, may be controlled.
Preferably, the sensor is a plasma arc torch positioned adjacent to a welder for welding the workpiece. The plasma arc torch includes a non-transferred pilot arc for ionizing a plasma gas to generate the plasma jet. A second motive device is also provided for moving the welder in a second direction, with the first direction being substantially transverse to the second direction. In one embodiment, the reference characteristic is a change in voltage across a circuit including the plasma jet. The apparatus also preferably includes a controller, such as a computer or other processor, for controlling at least the second direction based on an observed change in voltage. The physical characteristic of the workpiece is preferably the location of a seam, and the first direction of travel is substantially transverse to the seam. The first motive device also includes a motor for laterally translating a support for the plasma arc torch to and fro along the first direction of travel. Instead of a seam, the physical characteristic of the workpiece may also be the presence of an element positioned on the workpiece (i.e., a plate or the like) or an edge of the workpiece.
In an alternate embodiment, the welder and plasma arc torch are concentric, and the plasma arc torch rotates at least partially around the welder to provide sensing function. Specifically, a second motive device is provided for moving the welder along the workpiece in a second direction, and a second motive device is provided for rotating the plasma arc torch at least partially around the welder.
In accordance with a second aspect of the present invention, a system for automatically welding a workpiece is provided. The system comprises a welder for welding the workpiece, a sensor coupled to the welder for directing a plasma jet toward the workpiece, a first motive device for moving the sensor in a first direction, a second motive device for moving at least one of the welder or the workpiece in a second variable direction, with the first direction being substantially transverse to the second direction, and a controller, such as a computer or other processor, for controlling the second direction based on sensed changes in a reference characteristic of the plasma jet. The reference characteristic is preferably a voltage across a sensing circuit including at least the plasma jet as a resistive element. Preferably, the plasma jet is generated by a plasma arc torch having a non-transferred pilot arc positioned in advance of the welder. The sensor is preferably positioned in advance of the welder, and the two may be concentric.
In accordance with a third aspect of the present invention, a method of sensing a physical characteristic of a workpiece is disclosed. The method includes directing a plasma jet towards the workpiece, moving at least one of the plasma jet or the workpiece, sensing a change in a reference characteristic of the plasma jet, and sensing a physical characteristic of the workpiece based on the sensed change in the reference characteristic of the plasma jet. In a preferred embodiment, the plasma jet is established by a plasma arc torch, and the method further includes positioning the plasma jet torch in advance of a welder for welding a seam on the workpiece. Also, the step of moving is preferably moving the plasma jet and includes transversely scanning the plasma jet torch across the seam. The reference characteristic is preferably a voltage across a circuit including at least the plasma jet as a resistive element, and the method further includes determining a direction of travel for moving the welder along the seam based on changes in the sensed voltage. In one embodiment, the welder and plasma arc torch are concentric, and scanning is completed by rotating the plasma arc torch at least partially around the welder. The location of the seam is then determined based on the sensed changes in the reference voltage.
FIG. 1 is a schematic diagram showing an exemplary setup of a system for performing automated welding that uses a plasma arc torch as a sensor for creating a plasma jet, a characteristic of which is observed to sense a physical characteristic of the workpiece, such as the location of a weld seam;
FIG. 2a is a partially cutaway, partially schematic diagram of a plasma arc torch wherein a non-transferred arc is used to ionize a plasma gas and create a plasma jet and direct the jet toward the workpiece;
FIG. 2b is a partially cutaway, partially schematic diagram of a plasma arc torch wherein a transferred arc is used to ionize a plasma gas to create a transferred plasma arc;
FIG. 3 is a partially cutaway, partially schematic diagram of a plasma arc torch having a non-transferred arc used in the most preferred embodiment, also illustrating in particular the manner in which main and sensor power sources are connected to the sensor or plasma arc torch and the workpiece in the most preferred embodiment;
FIG. 4a is a graphical representation of an output reference voltage over time of a sensor circuit including at least the plasma jet as a resistive element when scanning a root opening;
FIG. 4b is a graphical representation of an output reference voltage over time of a sensor circuit including at least the plasma jet as a resistive element when scanning a substantially flat plate placed on a workpiece;
FIG. 5 shows an alternate embodiment of the sensor of the present invention.